In recent years, amorphous plastics including an acrylic resin, a methacrylic resin, a styrene resin, a polycarbonate resin and a cyclic polyolefin resin are being applied to various purposes, and there are large demands for optical materials, such as an optical lens and an optical disk substrate, owing to the optical characteristics thereof. Optical materials of the kinds requires not only high transparency, but also high performance that is well balanced among high heat resistance, low water absorbing property, mechanical properties and the like.
The materials having been conventionally used do not satisfy all the requirement but have problems to be solved. For example, polystyrene has problems of mechanical brittleness, large birefringence and deteriorated transparency. Polycarbonate has large birefringence and has transparency equivalent to polystyrene although it is excellent in heat resistance. Polymethyl methacrylate has high transparency, but is considerably high in water absorbance, which provides problems of poor dimensional stability and low heat resistance. Polyvinylcyclohexane, which is obtained by nuclear hydrogenation of polystyrene, is excellent in transparency, but has problems of low mechanical strength, poor heat resistant stability and poor adhesion property to other materials (see Patent Documents 1 to 3). A method of improving the adhesion includes a method of mixing nuclear-hydrogenated polystyrene, a product obtained by hydrogenating a double bond and an aromatic ring of a conjugated diene-polystyrene, or a saturated hydrocarbon resin (see Patent Document 4), but the method includes complicated operations. Such a method is disclosed that an aromatic vinyl compound, such as styrene, and an unsaturated dibasic acid, such as maleic anhydride, are copolymerized, and 30% or more of the aromatic rings are nuclear-hydrogenated (see Patent Document 5), but the transparency and birefringence thereof are improved as compared to polystyrene, but the optical characteristics are inferior to an acrylic resin. A copolymer of methyl methacrylate (which is hereinafter referred to as MMA) and styrene (which is hereinafter referred to as an MS resin) has high transparency and is excellent in balance among dimensional stability, rigidity, specific gravity and the like, but has a problem of large birefringence. It has been confirmed that a resin obtained by nuclear-hydrogenating an MS resin (which is hereinafter referred to as MSH) is excellent in balance among low birefringence, low water absorbing property, transparency, heat resistance, mechanical properties, weather resistance, light resistance and the like (see Patent Document 6).
Nuclear hydrogenation of an aromatic polymer has been known (see Patent Document 7). It is considered that the nuclear hydrogenation rate is necessarily increased for providing a resin having high transparency, and a highly transparent resin cannot be obtained unless the nuclear hydrogenation rate is substantially 100%. This is because a low nuclear hydrogenation rate provides a block polymer, which impairs the total light transmittance. Not only hydrogenation of an aromatic polymer but hydrogenation of such a polymer as a conjugated diene polymer has been numerously known, and a metal, such as Pd, Pt, Rh, Ru, Re and Ni, supported on a carrier, such as activated carbon, aluminum oxide, silica and diatom earth, is mainly employed. However, it has been known that the reaction is difficultly performed due to the high molecular weight, a high nuclear hydrogenation rate and a large reaction speed are difficultly obtained, and the catalyst activity is liable to be lowered upon repeated reaction. For avoiding the drawbacks, the catalyst carrier is regulated in kind, fine pore structure and particle diameter. For example, a method of providing nuclear-hydrogenated polystyrene having a nuclear hydrogenation rate of approximately 70% by using a supported palladium catalyst supported on a silica carrier of less than 100 μm (see Patent Document 1), and a method of providing nuclear-hydrogenated polystyrene with a platinum catalyst or a rhodium catalyst supported on a silica carrier having large pores exceeding 600 Å (see Patent Document 8) have been known. Such a method has been similarly known that a catalyst, which is obtained by supporting a Group VIII metal on a porous carrier having fine pores of 450 Å occupying 95% or more of the pore volume with the metal surface area thereof being 75% or less of the carrier surface area, is used, the hydrogenation rate of the aromatic moiety is suppressed to a low value, and the ethylenic unsaturated moiety is hydrogenated at a high hydrogenation rate (see Patent Document 9).
There are a literature disclosing that polystyrene is completely hydrogenated without decrease in molecular weight in the case where a Group VIII metal supported on silica or aluminum oxide has a pore volume for a pore diameter of from 100 to 1,000 Å of from 70 to 25% of the total pore volume (see Patent Document 10), and a literature disclosing that polystyrene is completely hydrogenated without decrease in molecular weight by using a commercially available hydrogenated catalyst for a low molecular weight compound, which is supported on an aluminum oxide carrier and has a pore volume for a pore diameter of from 100 to 1,000 Å of less than 15%, in the presence of an ether oxygen-containing hydrocarbon (see Patent Document 11). Furthermore, hydrogenation reaction of a conjugated diene system, such as an acrylonitrile-butadiene copolymer, is performed with a metallic catalyst of an oxide of a Group IVa element, such as titania and zirconium oxide, thereby providing high activity upon repeated use, but it is limited to a conjugated diene system, and there is no mention with respect to hydrogenation of an aromatic compound (see Patent Document 12). Such a method is disclosed that unsaturated bonds including aromatic rings of an aromatic compound-conjugated diene polymer can be efficiently hydrogenated, and elution of the metallic components is suppressed, with a catalyst obtained in a manner that an alkali metal or an alkaline earth metal is added to a carrier having a large fine pore volume, in which a pore volume for a pore diameter of from 100 to 100,000 nm occupies from 50 to 100% of the total pore volume, and then 90% or more of a platinum group component is supported on the surface layer within 1/10 from the outer surface in the depth direction based on the diameter of the carrier (see Patent Document 13).
A method of improving the reactivity by reacting the polymerization reaction solution as it is after removing the catalyst poison with activated alumina (see Patent Document 14), and a method of improving the reaction linear velocity in the fixed layer for enhancing the productivity (see Patent Document 15) are also disclosed.
The nuclear hydrogenation reaction in polymer reaction is largely influenced by a solvent, and in general, a hydrocarbon, an alcohol, an ether, an ester and the like are used as a reaction solvent (see Patent Document 16). However, there are problems that a hydrocarbon and an alcohol have a low resin solubility, and among ether compounds, for example, 1,4-dioxane has a low ignition point, and tetrahydrofuran is unstable since it is liable to undergo ring-opening reaction. An ester is safe and relatively stable and performs reaction rapidly, but has a problem of white turbidity of the resin depending on the nuclear hydrogenation rate. A method of providing a nuclear-hydrogenated aromatic polymer with high transparency safely, stably and rapidly by adding an alcohol to an ester is disclosed accordingly, but the combination use of two solvent systems makes separation operation complicated. A method of maintaining high transparency with a low nuclear hydrogenation rate by adding an alcohol or water to an ester solvent is disclosed (see Patent Document 17), but the method often cannot be employed since the cause thereof is not clear, and the application range thereof is limited.    [Patent Document 1] Japanese Patent No. 3,094,555    [Patent Document 2] JP-A-2004-149549    [Patent Document 3] JP-A-2003-138078    [Patent Document 4] Japanese Patent No. 2,725,402    [Patent Document 5] JP-B-7-94496    [Patent Document 6] JP-A-2006-89713    [Patent Document 7] West German Patent No. 1,131,885    [Patent Document 8] JP-T-11-504959    [Patent Document 9] Japanese Patent No. 3,200,057    [Patent Document 10] JP-T-2002-521509    [Patent Document 11] JP-T-2002-521508    [Patent Document 12] JP-A-1-213306    [Patent Document 13] JP-A-2000-95815    [Patent Document 14] JP-T-2003-529646    [Patent Document 15] JP-A-2002-249515    [Patent Document 16] JP-T-2001-527095    [Patent Document 17] Japanese Patent No. 2,890,748